Geological formations forming a reservoir for the accumulation of hydrocarbons in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed that may ingress or egress from the reservoir. To determine the behavior of the fluids in this network, knowledge of both the porosity and permeability of the geological formations is desired. From this information, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the resistivity of geological formations is a function of both porosity and permeability. Considering that hydrocarbons are electrically insulating and most water contain salts, which are highly conductive, resistivity measurements are a valuable tool in determining the presence of a hydrocarbon reservoir in the formations.
Electromagnetic (EM) surveying techniques have been used for characterizing geologic formations in the earth subsurface. An issue associated with EM surveying techniques is that wellbores lined with casings or liners (which are typically formed of steel) can adversely affect the accuracy of the surveying. The steel casing or liner has an effect on EM fields measured by EM receivers.
EM receivers measure electric and/or magnetic fields induced by EM sources. In a cross-borehole (CB) application, EM sources are placed in one wellbore and EM receivers are placed in a second wellbore. In a surface-to-borehole (STB) application, EM sources are placed on a land surface (or sea floor), whereas EM receivers are placed in a wellbore. In a borehole-to-surface (BTS) application, EM sources are placed in a wellbore, whereas EM receivers are placed on a land surface (or sea floor). In single-well (SW) borehole-to-borehole application, EM sources and receivers are both placed in the same wellbore. In each of the CB, STB, BTS and SW applications, a casing or liner may be present in a wellbore that contains the EM receivers and/or EM sources.
Another survey technique is a surface controlled source electromagnetic (CSEM) survey technique, in which both the EM sources and receivers are deployed on a surface (such as a sea floor or on land) within an area of interest to make measurements from which information about the earth subsurface can be derived. Note that the EM sources and/or receivers can also be towed in a body of water by a sea vessel. In the CSEM application, as well as in the STB and BTS applications, near-surface heterogeneity such as cultural structures including pipelines, metal fences, buildings, and geologic structures, can affect EM data measurements.
Removing effects of casings or liners, or near-surface heterogeneity, can improve image accuracy. Although various techniques have been proposed or implemented to reduce distortion caused by near-surface heterogeneity or removing effects of steel casing or liner in wellbores, it is noted that such techniques are subject to further improvement.